Assembly IA-32
Registers (Data)
Bits 31…24
Bits 23…16
Bits 15…8
Bits 7…0
AH
AL
EAX
AX
BH
EBX
BL
BX
DH
ECX
CL
CX
DH
EDX
DL
DX
Registers (Pointer and Index)
Bits 31…24
Bits 23…16
Bits 15…8
Bits 7…0
ESI
SI
EDI
DI
ESP
SP
EBP
BP
Registers (Control) – Flags & EIP
EIP – a 32bit register
that points to the next
instruction to be
executed
• Record information on the most recently executed arithmetic (e.g add, sub, etc) or
logical instruction (and, or, etc)
• Zero (ZF) – jz jnz
• Carry (CF) – jc jnc
• Overflow (OF) – jo jno
• Sign (SF) – js jns
• Parity (PF) – jp jnp
Flags (examples)
mov
sub
EAX, 8
EAX, 8
# ZF = 0
# ZF = 1
mov
cmp
EAX, 'a'
eax, 0
# ZF = 0
mov
cmp
EAX, '\0'
EAX, 0
# ZF = 1
cmp
EAX, EBX
# ZF = 1 if EAX = EBX
# for (i = 12# i != 0# i--)
mov
ECX, 12
# ECX = 12
loop:
# loop body
sub
ECX, 1
# ECX = ECX - 1
jnz
loop
# Jump if ZF = 0
mov
add
AL, 15
AL, 100 # SF = 0 (positive result)
mov
sub
AL, 15
AL, 100 # SF = 1 (negative result)
Registers (Segment)
• 16 bits:
•
•
•
•
•
•
CS (code segment)
DS (data segment)
SS (stack segment)
ES (extra data segment)
FS (extra data segment)
GS (extra data segment)
• Not used in modern applications
• Applications now use the flat memory model (unsegmented)
Addressing modes (1)
• Immediate
mov eax, 0xab
mov eax, 171
# eax = 171 hex: 0xab
# eax = 171 hex: 0xab
• Register Indirect Address
mov eax, esp
mov eax, [esp]
# eax = esp
# eax = *esp
• Index Addressing
mov reg, [reg + reg * scale + offset]
mov eax, [ebx + esi]
mov eax, [ebx + esi * 4]
mov eax, [ebx + esi * 4 + 10]
#scale = 1, 2, 4, 8,
#eax = *(ebx + esi)
#eax = *(ebx + esi * 4)
#eax = *(ebx + esi * 4 + 10)
Addressing modes (2)
• Memory set
mov [eax], ebx
mov [eax + 4], ecx
mov [eax + ebx], edx
mov [eax ], 4
# *eax = ebx
# *(eax+4) = ecx
# *(eax+ebx) = edx
# Error: operand must have the size specified
• PTR directive
mov byte ptr [eax], 2
mov word ptr [eax+2], 2
mov dword ptr [eax+4], 2
#*( ((char*)eax) ) = 2
#*( ((short*)eax) ) = 2
#*( ((int*)eax) ) = 2
Data Type
Size
Byte
8 bits
Word
16 bits
Double Word
32 bits
Labels and Comments
• Comments in assembly start with # and are single line
• Labels, are used as jump targets, they define a source location
<label_name>:
infinite_loop:
jmp Infinite_loop
some_label:
cmp eax, 0
je some_label
#jumps to some_label if eax is 0
Exercise 1
Translate the following C code to assembly:
# r = (a + b) – (c + d)
# r should be stored in eax, a,b,c,d values are on ebx, ecx, edx and esi respectively
Exercise 1 (Solution)
Translate the following C code to assembly:
# r = (a + b) – (c + d)
# r should be stored in eax, a,b,c,d values are on ebx, ecx, edx and esi respectively
add ebx, ecx
add edx, esi
# (ebx = ebx + ecx) | (ebx = a + b)
# (edx = edx + esi) | (edx = c + d)
mov eax, ebx
sub eax, edx
# eax = ebx
# eax = eax - ebx
| eax = a+b
| eax = (a+b)-(c+d)
Exercise 2
Translate the following C code to assembly:
#int * p;
#*p = 2+3;
# p is on eax
Exercise 2 (Solution)
Translate the following C code to assembly:
#int * p#
#*p = 2+3#
# p is on eax
mov ebx, 2
add ebx, 3
mov [eax],ebx
# ebx = 2
# ebx = ebx + 3
# *eax = ebx
Exercise 3
Translate the following C code to assembly:
# int max#
# if(a >= b)
# max = a#
# else
# max = b#
# max should be stored on eax, a and b are on ebx and ecx respectively
Exercise 3 (Solution)
Translate the following C code to assembly:
# int max#
# if(a >= b)
# max = a#
# else
# max = b#
# max should be stored on eax, a and b are on ebx and ecx respectively
cmp ebx, ecx
jl max_ecx
mov eax, ebx
jmp end
# update_flags(ecx – ebx)
# go to max_ecx if ebx < ecx
# eax = ebx
# goto end
mov eax, ecx
# eax = ecx
max_ecx:
end:
Exercise 4
Translate the following C code to assembly:
#int sum = 0
#for(int i = 0; i < 10; ++i)
#
sum+=i;
# sum should be stored on eax
Exercise 4 (Solution)
Translate the following C code to assembly:
#int sum = 0#
#for(int i = 0# i < 10# ++i)
#
sum+=i#
# sum should be stored on eax
mov eax, 0
# eax = 0
mov ebx, 0
# ebx = 0
#for
loop:
cmp ebx, 10
jnl end_loop
add eax, ebx
inc ebx
jmp loop
end_loop:
# update_flags(10 - ebx)
# jump if not less
# eax = eax + ebx
# ebx++#
# goto loop